/Engine/Source/Runtime/Experimental/Iris/Core/Public/Iris/ReplicationSystem/RepTag.h
The invalid RepTag can be used for purposes where one wants to know if there's a valid tag or not. It cannot be a constant like RepTag_Invalid as that would require us to prevent a tag from being called Invalid or cause mismatching values if someone calls a tag Invalid. So it's implemented as a function instead.
namespace UE { namespace Net { constexpr [FRepTag](API\Runtime\IrisCore\Iris\ReplicationState\FRepTag) UE::Net::GetInvalidRepTag() } }